Modular and prefabricated bridges are spans that are partially constructed at the supplier’s factory, rather than onsite like conventional bridges, before transportation to the building site. They are highly adaptable and suitable for many span lengths due to their ease of assembly and lower costs. They may be better-constructed as they are built indoors by a trained specialized work force using more sophisticated tools than are usually found in open air jobsites. The industry of modular bridge building originated from military engineering, or pioneering. On the battlefield, engineers moved soliders and supplies by quickly erecting strong bridges over water using simple pontoons with little time to survey, excavate, pour concrete, or make footings.

Most prefabricated bridges are simple truss designs, with their side railings also the supporting truss. The side rails are the single most distinguishing feature of a Bailey type of prefab bridge, as shown below.

A truss bridge’s side rails are also safer, lowering the chance a vehicle or person will go off the side. These bridges can be used indefinitely for rural wilderness areas, pedestrian walkways, and bikeways. But for normal automobile traffic at normal speeds, they must eventually be replaced with a permanent fixed bridge.

The nature and ease of construction and emplacement allows for easy field modifications for the special needs of the particular modular bridge. The bolted modular design method commonly used in wartime lends itself well to this. In peacetime, however, builders can usually call on heavy equipment to hoist the span sections into place, welders for extra support pieces, and concrete machinery and supplies to build the piers and footings.

Operation and Specifications

Ideally, basic surveying and site work such as approaches, piers and footings need to be performed, cleared, poured, and emplaced beforehand. Properly specified bridges are safe for immediate use, but considering its bolted and pinned construction, it must be frequently checked and tightened than a conventional bridge during comparable use.

A bridge’s most important specification is the total length needed to be spanned, then the width of the span. The span lengths usually vary from 30′ to 200’, and widths are usually limited to two lanes of traffic. Within this context builders can select a span length that requires no piers, bottom supports, or pontoons. Next is the load the bridge span must bear, which will then set the type of deck. Steel grate, paved, or wood decks are common. If more than one standardized section is needed, an arch must be used or a support provided below such as a pier or pontoon.

Bailey truss style bridges using a common steel alloy are one of the most common designs. Combination of a truss and arch, which gives the span more support, is also a frequent design. An example of a truss and arch combination is shown in the image above.

Image credits:

Fmiser / CC BY-SA 3.0 | Pioneer Bridges